A frontlight unit is a means for illuminating a display assembly, such as a liquid crystal display (LCD), an interferometric modulation display (IMOD), or a display that is viewed in ambient light. A provision of the frontlight unit improves performance of the display in poor lighting conditions. Frontlights are inferior to backlights and are usually used as an auxiliary feature.
Generally, a display presents an image by absorbing some of the light passing through. When an electrical field is applied across the crystal, e.g., in an LCD, the electrical field changes the passing light so that light does not pass through a polarization filter. When the electrical field is absent, the polarization filter passes predetermined wavelength components of white light. The remaining portion of white light is absorbed. For example, in an RGB image obtained from white light, approximately two-thirds of the light energy is lost and does not participate in the image-formation process. The aforementioned absorbed light is produced by a power source, e.g., a battery-operated device. If it were possible to obviate the losses of light energy associated with the absorption of the light components that do not participate in the image-formation process, it would be possible to significantly improve power-source efficiency, i.e., to prolong the service life of the light source, such as a battery.
A nonilluminated display may be lit from the front. To use ambient light in an LCD, the liquid crystal, itself, is sandwiched between a polarization filter and a reflective surface, i.e., a mirror. The mirror causes the display to be opaque so that the display cannot be illuminated from the back. In order to improve the brightness and contrast of the image reproduced by a reflective display, it is desirable to enhance illumination with an additional artificial source of frontlight.
Frontlight systems for reflective displays are relatively new devices, and the concept of frontlight display is also relatively new. Nevertheless, many patents and patent applications are already dedicated to the structure and use of frontlight devices (see, e.g., U.S. Pat. No. 6,650,382 issued on Nov. 18, 2003 [inventors: Sumida, et al]; U. S. Pat. No. 6,657,683 issued on Dec. 2, 2003 [Inventor: Richard]; U.S. Pat. No. 7,603,001 issued on Oct. 13, 2009 to Wang, et al; U.S. Pat. No. 7,502,081 issued on Mar. 10, 2009 to Umemoto, et al; U.S. Pat. No. 7,733,439 issued on Jun. 8, 2010 to Sampsell, et al; U.S. Pat. No. 7,777,954 issued on Aug. 17, 2010 to Gruhlke, et al; U.S. Pat. No. 7,855,827 issued on Dec. 21, 2010 to Xu, et al; U.S. Pat. No. 7,813,026 issued on Oct. 12, 2010 to Sampsell; U.S. Pat. No. 7,864,395 issued on Jan. 4, 2011 to Chui; U.S. Pat. No. 8,009,244 issued on Aug. 30, 2011 to Toriyama, et al; U.S. Pat. No. 7,944,524 issued on May 17, 2011 to Akiyama, et al; and U.S. Pat. No. 7,949,213 issued on May 24, 2011 to Mienko, et al).
Various methods for delivery of frontlight are known in the art. Most often, a light source is placed around the perimeter of an LCD. Other systems use backlight, which is redirected to the display edges and which then propagates through the space between the display, itself, and the front protective layer. There exists a great variety of improvements to both of these concepts, such as preventing exit of propagated light from the aforementioned space, or similar methods aimed at providing full reflection of light in said space. Other methods and means are aimed at improving efficiency of frontlight-separation, e.g., improving input of light from display edges, etc.
Common drawbacks of existing frontlight display illumination systems are inefficient use of light source, insufficient brightness and contrast, and complexity of structure.
Implementation of the holographic technique for frontlight display illumination is also known in the art. U.S. Pat. No. 7,845,841 issued on Dec. 7, 2010 to J. Sampsell discloses a frontlight display illuminator that uses holograms embedded in a waveguide plate of a special configuration.
U.S. Pat. No. 7,859,731 issued on Dec. 28, 2010 to Jin-Seung Choi discloses an illumination apparatus and method for a display device designed such that light is incident on a hologram or hologram pattern at an angle for which diffraction efficiency is the highest. The illumination apparatus includes at least one point of light source that emits light and a light guide plate (LGP) that has at least one point of light source disposed on a side thereof and a hologram pattern on the top surface that permits the light incident from the point of light source to exit from the top surface. The side of the LGP facing the point of light source is inclined such that the light is incident obliquely on the hologram pattern at an altitude angle that provides high diffraction efficiency.
The use of lasers for backlighting is known. For example, U.S. Pat. No. 7,508,466 issued on May 24, 2009 to Hutchins discloses an LCD display that includes a planar array of transmissive LCD devices and at least one laser diode device spaced apart from the planar array of LCD devices. At least one laser diode device is configured to illuminate at least a subset of LCD devices of the planar array of LCD devices so that in operation the laser diode device provides backlighting for the subset of LCD devices.
US Patent Application Publication No. 20110026270 published on Feb. 3, 2011 (inventor: Onishi) discloses a surface light-source device from which a large planar light with uniform light intensity distribution can be obtained from a spot-like laser light. The surface light-source device comprises a laser light source for emitting laser light, an optical system including one or more reflective diffusion members, and an optical waveguide combined with the optical system for converting reflected and diffused laser light into the planar light and emitting it from a principal plane. The reflective diffusion member converts the laser light emitted from the laser light source into a linear light having an arcuate radiation pattern.
US Patent Application Publication No. 20080259247 (published on Oct. 23, 2008, inventor: C. Stuart, et al) discloses a display, such as an LCD panel, that is illuminated using frequency-doubled vertical extended-cavity surface-emitting lasers (VECSELs) as efficient light sources. Visible light from VECSELs is directed to an illuminating panel using optical fibers and/or optical gratings to provide substantially uniform illumination of the illuminating panel. Visible light from the illuminating panel, which can be provided at a particular number of primary wavelengths by the VECSELs, is then used to illuminate the display.
A laser light source supplied by special optics for use in planar lighting devices as a backlight device is also known in the art. U.S. Pat. No. 7,859,610 granted on Dec. 28, 2010 to T. Mizushima describes a laser system that can be used for RGB laser light illumination in display assemblies.
Further development of front illumination systems with improved efficiency of lighting is disclosed in U.S. patent application Ser. No. 13/373,434 filed by the same applicants as the applicants of the present application on Nov. 14, 2011. The system comprises a frontlight unit that is intended to enhance illumination of a reflective display having pixels arranged in a matrix pattern and using monochromatic laser lights as light sources. The unit contains a network of light-distribution planar ridge waveguides with holograms arranged in a matrix pattern that corresponds to the matrix pattern of the reflective display when the frontlight unit is interposed onto the reflective display. The unit emits light in the downward direction in the form of diverging beams that fall onto the pixels of the reflective display and in the upward direction onto mirrors from which light is reflected also in the form of diverging beams onto the reflective display. However, the reflective mirrors occupy up to 10% of the surface area of the display, thus blocking a portion of image-carrying light.